1. Field of the Invention
The present invention relates to a hybrid construction machine which utilizes a combination of power from an engine and electric power from a battery.
2. Description of the Background Art
The hybrid-type shovel is exemplified to describe the background art.
Hybrid-type shovels comprise, as shown in FIG. 5, a lower propelling body 1, an upper slewing body 2 which is mounted on the lower propelling body 1 to be capable of slewing around an X-axis perpendicular to the ground and a work attachment 6 connected to the upper slewing body 2. The work attachment 6 comprises a boom 3, an arm 4 and a bucket 5.
Hybrid-type shovels also comprise an engine as a power source, a hydraulic pump and a generator motor which are connected to the engine and a battery. The hydraulic pump drives a hydraulic actuator by discharging hydraulic fluid by power from the engine. The generator motor acts as a generator by receiving power from the engine and electric power generated by this action is charged on the battery. The generator motor also acts as an electric motor by receiving electric power from the battery, as appropriate, to assist the engine.
Hybrid-type shovels also comprise a slewing electric motor (work electric motor) as a slewing driving source for the upper slewing body 2. The slewing electric motor is driven by electric power from the battery. Meanwhile, the slewing electric motor acts as a generator when slewing speed is reduced and regenerative electric power generated by this action is charged on the battery.
In the hybrid-type shovels, the upper limit of battery charge-discharge electric current is determined based on “present charging rate” (usually referred to as “SOC [State Of Charge]”) which is the currently remaining charged amount of the battery and this upper limit is used for various controls. Therefore, in the hybrid-type shovels, it is necessary to always monitor and know the present charging rate.
Japanese Examined Patent Publication No. S59-8789 (hereinafter referred to as patent literature 1) discloses a calculation method of the present charging rate. More specifically, the calculation method disclosed in patent literature 1 comprises integrating charge current and discharge current from a battery to calculate the quantities of charged electricity and discharged electricity, and adding or subtracting the above quantities of electricity to or from an initial capacity (charging rate at the start point of use of the battery) of the battery to calculate the remaining capacity (present charge rate) of the battery. Namely, the calculation method disclosed in patent literature 1 is the one based on so-called “integration of electric current” (hereinafter also referred to as “current integration method”).
Japanese Patent Application Publication No. 2001-281306 (hereinafter referred to as patent literature 2) discloses a calculation method based on so-called “voltage reference” (hereinafter also referred to as “voltage reference method”). More specifically, the calculation method of patent literature 2 comprises determining the present charging rate from the detected terminal voltage of a battery based on a preliminarily stored map for the relation between a terminal voltage and a charging rate of the battery.
In the current integration method, electric current flowed in and out from a battery is integrated regardless of the fluctuation in voltage of the battery upon charge and discharge. Due to this, the current integration method is suitable for calculation during operation of a machine (during battery is charged and discharged), while it has a drawback that it is difficult to accurately calculate the present charging rate because of accumulation of calculation errors due to the sensor tolerance and battery deterioration.
On the other hand, the voltage reference method only requires detection of voltage of a battery. Therefore, the voltage reference method is less affected by the sensor tolerance etc., while it is not suitable for calculation during operation of a machine because the calculation accuracy is reduced when voltage of the battery is significantly fluctuated due to charge and discharge of the battery during operation of the machine.
As described above, both calculation methods have merits and demerits and it is difficult to accurately calculate the present charging rate of a battery depending on operation status of a machine by either methods. Thus, there have been problems such that it is not possible to take full performance of a battery and that a shovel cannot be appropriately controlled.